The image scanning apparatus has been widely used nowadays in many applications. In the recent, apparatus capable of scanning image in both the reflective mode and transparent mode was introduced into the market. The basic principle and detailed structure of the scanning apparatus of the type mentioned may be referred to the U.S. Pat. No. 4,879,604.
The lamp used to illuminate the original having image thereon may be a hot cathode fluorescent lamp or a cold cathode fluorescent lamp widely used in office automation equipments, e.g. facsimile, xerox machine or scanner.
Like other types of the lamp, the quality of the lamp used in the image scanning apparatus and, therefore, the intensity of light illuminated from the lamp deteriorates as the lamp operates for a certain amount of time. Due to the aforesaid reason, the quality of the output of the image scanning operation is worse than that when the lamp is a brand new one.
On the other hand, the light transmissivity of different original bearing image thereon may vary. A light source of preset intensity will likely be too bright for different original and the result of the scanning process will be poor. Under this kind of condition, the intensity of light irradiated by the lamp must be reduced to a desired level.